Lipoic acid intermediates



United States Patent LIPOIC ACID INTERMEDIATES Lester J. Reed, Austin, Tex., assignor to Research Corporation, New York, N.Y., a corporation of New York No Drawing. Filed July 8, 1954, Ser. No. 442,419

1 Claim. (Cl. 260-399) This invention relates to intermediates useful in the production of lipoic acid and to method of making them. It relates particularly to 6,S-dibenzylmercaptooctanoic acid of the formula The acid is obtained by reacting a 6,8-dihalooctanoic acid or ester with benzylrnercaptan in the presence of an alkali metal hydroxide or alcoholate. The acid can be converted to 6,8-dithioloctanoic acid by means of an alkali metal in liquid ammonia. The latter is readily oxidized to a-lipoic acid.

Referring more particularly to the following equations, illustrating the principles of the invention, in which R represents an esterifying alkyl or aralkyl group, 8- chloro-6-oxooctanoic esters (I) may be reduced with an alkali metal boron hydride to the 8-chloro-6-hydroxy ester (XII) and the latter converted to the 8-chloro-6- halo compound (XIII) with a suitable halogenating agent, such as thionyl chloride or phosphorus tribromide.

The 8-chloro-6-oxooctanoic acid ester may be converted to the 6-oxo-7-octenoic ester (II) by heating. The 6-oxo-7-octenoic ester may be converted to 8-bromo-6- oxo-octanoic ester (III) by the addition of HBr and further converted to the 6,8-dihalooctanoic ester (V) by reduction of the 0X0 group to hydroxyl (IV) with an ltali metal boron hydride followed by treatment with a halo-genating agent to replace the hydroxyl with halogen.

The same overall conversion may be accomplished by first reducing the oxo group of the 6-oxo-7-octenoic ester to hydroxyl, hydrolyzing the ester to the tree fi-hydroxy- 7-octenoic acid (IX), treating with HBr to form the 6,8dibromooctenoic acid (X) and esterifying the acid.

H C10 (CHDACOQR CH CHz l AlCls lHBr II BICHQCHZC (CH2)4C 02R (III) lNaB H4 I 1 BrCHzCHzCH(CH2)4C02R I (V) KOHl I llTz-HKI (1) NaBH4 (2) NaOH lHIBr l CH2N2 lNaB H4 oiomomhmonnroorn x11 lSOCh (2) KOH l Na-liq.NHa

VII

. Patented Mar..14, 1961 p vigorous Stirring, 70 g. of ethyl 6-chloroformylvalerate (H. Bergs, C. Wittfeld and H. Frank, Ber., 67b, 1622 (1947)). The temperature is maintained at 25 C. The cooling bath is removed and ethylene is passed in for a period of 2 hours. The reaction mixture is poured onto cracked ice, the organic layer separated, and the aqueous layer extracted with 200 ml. of chloroform. The combined organic extracts are dried over anhydrous sodium sulfate and the solvent removed in vacuo. The darkcolored oil remaining, crude ethyl 8-chloro-6-oxooctanoate I), is distilled in vacuo through a 6 in. Vigreaux column. After a small forerun, the main fraction (II), 48-54 g. (72-80%), B.P. 112-114 C. (2 mm.); 11 1.4485, is collected.

Ethyl 8-bromo6-hydr0xy0ctan0ate (IV).--A solution of 51.3 g. of ethyl 6-oXo-7-octenoate (II) and 50 ml. of reagent-grade benzene is cooled in an ice-bath and shaken intermittently while anhydrous hydrogen bromide is passed into the solution. When 24.8 g. of hydrogen bromide has been absorbed, the flask is stoppered and allowed to stand at room temperature for 20 hours. The organic solvent is removed in vacuo and the reddish oil, crude ethyl 8-bromo-6-oxooctanoate (III), is dissolved in 200 ml. of 95% ethanol. This solution is maintained at 20 C. while a solution of 5.3 g. of sodium borohydride in 20 ml. of water is added dropwise with stirring (approximately 10 minutes). Forty milliliters of concentrated ammonium hydroxide is added, the cooling bath is removed, and stirring is continued for 30 minutes at room temperature. The reaction mixture is poured into 500 ml. of water and extracted with two 150 ml. portions of ether. The combined ether extracts are washed with 50 ml. of 5% hydrochloric acid and dried over anhydrous sodium sulfate. The organic solvent is removed in vacuo and the oily residue distilled through a 6 in. Vigreaux column. After a small forerun (ca. 5 g.), the main fraction is collected, 40.8-47.5 g. (55-64% B.P. l32-l34 C. (0.5 mm.), n 1.4761; on redistillation it boiled at 132 C. (0.5 mm.), n 1.4767.

Ether may also be used as a solvent in the reaction with hydrogen bromide.

Ethyl 6,8-dibromooctanate (V) .-A solution of 86 g. of ethyl 8-bromo-6-hydroxyoctanoate (IV) and 50 ml. of reagent-grade carbon tetrachloride is maintained below 0 C. while 32 g. of phosphorus tribromide is added dropwise with stirring (approximately 25 minutes). Stirring is continued for 2 hours at 0 and the reaction mixture is allowed to stand at room temperature for 16 hours. At this time the reaction mixture is cooled to 0 and 70 m1. of water is added dropwise with stirring. Two hundred milliliters of ether is added and the mixture shaken in a separatory funnel. The organic layer is Washed successively with two 100 ml. portions of sodium bicarbonate solution and 50 ml. of Water, and then dried over anhydrous sodium sulfate. The organic solvent is removed in vacuo and the oily residue distilled through a 6 in. Vigreaux column. A small forerun (2-3 g.) is collected, followed by the main fraction, 55.2-59.5 g. (52-56%), B.P. 119-121 (0.5 mm.), n 1.4930; on redistillation, B.P. 119 (0.4 mm.), 12 1.4938.

Ether may be used as a solvent in place of carbon tetrachloride.

DL-a-lipoic acid (VIII).Redistilled thiolacetic acid (14.7 g.) is cooled in an ice-bath and neutralized to the phenolphthalein end-point with a 10% solution of potassium hydroxide in ethanol (approximately 135 ml. required). To this solution is added 29 g. of ethyl 6,8- dibromooctanoate (V) and the mixture is stirred and heated under reflux in an atmosphere of nitrogen for 5 hours. The reaction mixture, which contains ethyl 6,8- diacetylmercaptooctanoate (VI), is cooled and 35 g. of potassium hydroxide (85%) is added. The reaction mixture is stirred at room temperature in an atmosphere of nitrogen for 17 hours, then acidified (ph 1) with Cir 4 6 N hydrochloric acid. Ethanol is removed in vacuo, sufiicient Water is added to dissolve the inorganic solids and the mixture is extracted with two 150 ml. portions of chloroform. To the combined organic extracts, which contain 6,8-dimercaptooctanoic acid (VII), is added 575 ml. of chloroform and 210 ml. of water. This mixture is stirred vigorously in an atmosphere of nitrogen while sufiicient iodoform reagent (R. L. Shriner and R. C. Fuson, Identification of Organic Compounds, 2nd ed., John Wiley and Sons, New York, N.Y., 1940, p. 53) is added dropwise during a 6 hour period to give a permanent brown color. Approximately 185 ml. of iodoform reagent is required. The organic layer is separated, Washed with 500 ml. of 1% sodium thiosulfate solution, and then extracted with two 250 ml. portions of 5% sodium bicarbonate solution. The aqueous extracts are acidified (pH 1) with 6 N hydrochloric acid and extracted with two 125 ml. portions of chloroform. The combined chloroform extracts are dried over anhydrous sodium sulfate and the solvent is then removed in vacuo.

The yellow viscous oil remaining solidifies when cooled and scratched. This solid material is extracted with three 300 ml. portions of boiling Skelly B solvent (essentially n-hexane). The combined extracts are seeded with crystalline DL-a-lipoic acid and allowed to stand at room temperature overnight and then in a refrigerator for several hours. Large yellow crystals separate, M.P. 60.5- 61.5 C. The yield of product is 10.8-12.3 g. (60-68%). It possesses the characteristic ultraviolet absorption spectrum of Ot-llPOlC acid: A max. 332, e max. 144; A min. 280, 6 min. 31. On recrystallizing from Skelly B solvent, M.P. 61-62 C.

Methyl 6,8-'dibr0m0octan0ate (XI).A solution of 34.5 g. of ethyl 6-oxo-7-octenoate (II) in ml. of methanol is maintained at 20 C. while a solution of 2.12 g. of sodium borohydride in 25 ml. of methanol is added dropwise with stirring. The reaction mixture is allowed to stand 2 hours at room temperature, and then cooled in an ice-bath. Sodium hydroxide (22.4 g.) is added with stirring. When the sodium hydroxide has dissolved, the cooling bath is removed and stirring is continued at room temperature for 13 hours. The reaction mixture is acidified (pH 1) with 6 N I-ICl and the methanol is removed in vacuo. The reaction mixture is extracted with two 150 ml. portions of ether. The combined ether extracts are shaken with three 100 ml. portions of 5% sodium bicarbonate solution. The combined bicarbonate extracts are acidified (pH 1) with 6 N HCl and the mixture is extracted with three 100 ml. portions of ether. The combined ether extracts are dried over anhydrous sodium sulfate and the ether is removed in vacuo. Crude 6-hydroxy-7-octenoic acid (IX), 23.3 g., remains as a viscous oil.

To this product is added 350 mg. of benzoyl peroxide and the mixture is saturated with anhydrous hydrogen bromide. The flask is stoppered and allowed to stand at room temperature overnight. The reaction mixture, which contains 6,8-dibromooctanoic acid (X), is dissolved in 200 ml. of ether and washed with three 50 ml. portions of water. An ether solution (340 ml.) of diazomethane, prepared from 34.4 g. of N-nitrosomethylurea (F. Arndt, Org. Synthesis, 2, 461 1948)) is added slowly in the cold. The reaction mixture is allowed to stand 1 hour at room temperature and then concentrated in vacuo to remove excess diazomethane. The solution is dried over anhydrous sodium sulfate and the ether is removed in vacuo to yield 32 g. of an oily residue, which is distilled in vacuo from a modified Claisen flask. Three fractions are collected: 8.1 g., B.P. 93-98 C. (0.3 mm.); 4.4 g., B.P. 98-1l3 C. (0.3 mm.); 13.2 g., B.P. 114-118 C. (0.2 mm.), 11 1.4927. The third fraction represents a 29% yield of methyl 6,8-dibromooctanoate, based on the amount of ethyl 6-oxo-7-octenoate used; on redistillation, B.P. -112 C. (0.1 mm.); 11 1.4951.

Methyl 6,8-dibromooctanoate is converted to DL-ulipoic acid in 64% yield by the same procedure as employed with ethyl 6,8-dibromooctanoate.

Ethyl 8-chloro-d-hydroxyoctanoate (XII) .-To a suspension of 108 g. of aluminum chloride in 450 ml. of carbon tetrachloride is added dropwise, with vigorous stirring, 72 g. of ethyl fi-chloroformylvalerate. The temperature is maintained at 25 C. The cooling bath is removed and ethylene is passed in fora period of 2 hours. The reaction mixture is poured onto cracked ice, the organic layer separated, and the aqueous layer extracted with 200 ml. of chloroform. The combined organic extracts are dried over anhydrous sodium. sulfate and the solvent removed in vacuo. The dark-colored oil remaining is dissolved in 200 ml. of 95% ethanol and maintained at 20 C. with stirring while a solution of 7.06

g. of sodium borohydride in 20 ml. of water is added dropwise. Concentrated ammonium hydroxide '(40 ml.) is then added, the cooling bath is removed, and stirring is continued for 1 hour. The reaction mixture is poured into 500 ml. of water and extracted with two 150 ml. portions of ether. The combined ether extracts are washed with 50 ml. of 5% hydrochloric acid and dried over anhydrous sodium sulfate. The solvent is removed in vacuo and the oily residue distilled through a 6 inch Vigreaux column. A small forerun is collected, followed by the main fraction, 49-60 g. (59-72%), B.P. l2l-123 C. (0.5 mm.), 11 1.4573; on redistillation, B.P. 121 C. (0.4 mm.), 11 1.4580.

Ethyl 6,8-dichl0r00ctan0ate (XIII).-A solution of 29.2 g. of ethyl 8-chloro-6-hydroxyoctanoate (XII) in 28 ml. of anhydrous benzene is added dropwise with stirring to a solution of 18.5 g. of thionyl chloride in 14 m1. of benzene containing two drops of pyridine. The reaction mixture is then heated under gentle reflux for 1 hour, cooled, and shaken with 50 ml. of ice-water. The organic layer is separated, dried over anhydrous sodium sulfate and distilled in vacuo. A small forerun (4. g.) is collected, followed by the main fraction, 24.5 g. (77%), B.P. 100-103 C. (0.05 mm.), n 1.4600; on redistillation, B.P. 99.5-100 C. (0.05 mm.), n 1.4603.

6,8-dibenzylmercaptooctanoic acid (XIV).--

(Method A): To a solution of 12.4 g. of benzylmercaptan in 69 ml. of alcoholic potassium hydroxide is added 14.9 g. of ethyl 6,8-dibromooctanoate. The mixture is stirred and heated under reflux in an atmosphere of nitrogen for 3 hours. The mixture is cooled in an ice-bath, 8 g. of 85% potassium hydroxide is added, and the mixture is stirred at room temperature for 17 hours. The reaction mixture is acidified with 6 N hydrochloric acid, diluted with 200 ml. of water and extracted with two 100 ml. portions of ether. The combined ether extracts are treated with iodoform reagent until a permanent brown color is produced (approximately 7 ml. required). The ether layer is washed with 100 ml. of 1% sodium thiosulfate solution and then extracted with two 100 ml. portions of 5% sodium hydroxide solution. The aqueous layer is acidified with 6 N hydrochloric acid and extracted with 100 ml. of ether. The ether extract is dried over anhydrous sodium sulfate and the solvent removed in vacuo. A viscous oil remained which solidifies when cooled and scratched. The solid material is crystallized from a mixture of benzene and n-hexane to give 14.3 g. (84.6%) of colorless needles, M.P. 68-69 C.; on recrystallization from benzene-nhexane, M.P. 68.5-69.5 C.

(Method B): To 150 ml. of absolute ethanol is added 5.06 g. of sodium. When all of the sodium has reacted, 27.3 g. of benzyl mercaptan and 24.1 g. of ethyl 6,8-dichlorooctanoate are added. The mixture is stirred and heated under reflux in an atmosphere of nitrogen for 14 hours. The reaction mixture is cooled to room temperature, and 11.2 g. of potassium hydroxide is added. When the latter has dissolved, stirring is stopped and the reaction mixture is allowed to stand at room temperature for 20 hours. The reaction mixture is poured into 200 ml. of water, acidified with 6 N hydrochloric acid and extracted with two 150 ml. portions of ether. The com-. bined ether extracts are dried over anhydrous sodium sulfate and the solvent is then removed in vacuo. The viscous oil remaining is dissolved in 105 ml. of warm benzene and 400 ml. of hot Skelly B is added gradually. The clear solution is seeded with crystalline 6,8-dibenzylmercaptooctanoic acid and allowed to stand overnight in a refrigerator. The crystalline product is collected on a Biichner funnel and washed well with cold Skelly B. The yield of product is 31 g. M.P. 67.5-69" C.

6,8dimercapt0octanoic acid (VII).--To a stirred solution of 6.9 g. of sodium in 700 ml. of liquid ammonia is added dropwise a solution of 38.8 g. of 6,8-dibenzylmercaptooctanoic acid in 75 ml. of toluene.. Toward the end of the reaction, small pieces of sodium are added to maintain a permanent blue color. A total of 8.8 g. of sodium is employed. The ammonia is allowed to evaporate and 150 ml. of cold water is added to dissolve the solid residue. The organic layer is separated and discarded. The aqueous layer is made strongly acid with concentrated hydrochloric acid and extracted with three 50 ml. portions of chloroform. The combined organic extracts are dried over anhydrous sodium sulfate and the solvent is removed in vacuo. The oily residue is distilled under reduced pressure in an atmosphere of carbon dioxide. The total yield of 6,8-dimercaptooctanoic acid, B.P. 160162 C. (0.7 mm.), 71 1.5231 is 17.0 g. (82%).

DL-a-lipoic acid '(VIlI).-Sodium (2.64 g.) is added in small portions to 400 ml. of liquid ammonia. This solution is stirred while a solution of 11.2 g. of 6,8-dibenzylmercaptooctanoic acid in 50 ml. of anhydrous ether is added dropwise during a 30 minute period. The blue color is discharged with ammonium chloride and the ammonia is allowed to evaporate. The solid residue is extracted with 100 ml. of water and 50 ml. of ether.

The aqueous layer is acidified (pH 1) with 6 N hydrochloric acid and then extracted with three 50 ml. portions of chloroform. To the combined chloroform extracts is added 138 ml. of chloroform and 70ml. of water. This mixture is stirred vigorously in an atmosphere of nitrogen while sufiicient iodoform reagent (60 minute period to r The organic layer is ml.) is added dropwise during a give a permanent brown color. washed with ml. of 1% sodium thiosulfate solution and dried over anhydrous sodium sulfate. The solvent 0 is removed in vacuo and the yellow oil remaining is extracted with one 300 ml. portion and one 100 ml. portion of boiling Skelly B. The combined extracts are seeded with crystalline DL-a-lipoic acid and allowed to stand at room temperature overnight and then in a refrigerator for several hours. Large yellow crystals separate, M.P. r

6162 C. The yield of product was 4.85 g. (82%).

Another method of preparing DL-a-lipoic acid is as follows:

Sodium 3.45 g.) is added in small portions to 400 ml. of liquid ammonia. This solution is stirred while a solution of 19.4 g. of 6,8-dibenzylmercaptooctanoic acid in 40 ml. of toluene is added dropwise during a 30 minute period. Toward the end of the reaction, small pieces of sodium are added to maintain a permanent blue color. A total of 4.4 g. of sodium is employed. The bluecolor is discharged with ammonium chloride and the ammonia is allowed to evaporate. The residue is extracted with 100 ml. of cold water. The toluene layer is separated and discarded. The aqueous layer is made acid to phenolphthalein and acid to litmus with 6 N hydrochloric acid and diluted to 400 ml. Two milliliters of 1% ferric chloride is added and a rapid stream of oxygen is passed in from a sintered glass inlet tube until the color vofthe J solution changes to pale yellow (approximately N The solution is then acidified (pH 1) with: 6 N hydrochloric acid and extracted'with two. 100ml."

minutes b portions of chloroform. The combined organic extracts are dried over anhydrous sodium sulfate and the solvent is removed in vacuo. The viscous yellow oil remaining is extracted with one 400 ml. and one 200 ml. portion of boiling Skelly B.

The DL-a-lipoic acid is crystallized from the solvent as in the preceding method.

This application contains subject matter in common with my applications Ser. No. 436,255, filed June 11, 1954, and 442,418 and 442,420, filed July 8, 19 54.

I claim:

The method of making 6,8-dibenzylmercaptooctanoic acid which comprises heating a substance of the group consisting of acids of the formula HaICH CI-I CHHaM CH CO H wherein Hal is selected from bromine and chlorine and the lower alkyl esters thereof with benzylmercaptan in 8 the presence of an alkali metal compound of the group consisting of alkali metal hydroxides and alkali metal alcoholates.

References Cited in the tile of this patent UNITED STATES PATENTS 2,010,731 Lubbert et al -u Aug. 6, 1935 2,472,471 Eby June 7, 1949 10 2,789,991 Brockman et al Apr. 23, 1957 OTHER REFERENCES 

